Cutting elements having a polycrystalline diamond top surface are being utilized as the cutting or work portions of drilling or boring tools. Such cutting elements have been used in applications for drilling bore holes in subterranean formations in the mining, construction, oil and gas exploration, and the oil and gas production industries. There are many and varied forms and shapes of cutting elements currently being utilized with drill bits. One of the common insert shapes utilizes a cylindrical base section for insertion into the drill opening or socket of a drill bit body, with the upper or protruding portion of the cutting element being generally cylindrical with a planar polycrystalline diamond top surface. Many various shapes for the generally cylindrical upper or protruding section are in use.
Commercially available drill bits are classified as either roller bits or drag-type bits. A fixed cutter element is used as a part of the drag-type drill bits and do not employ a cutting structure with moving parts, for example, a rolling cone bit. The fixed cutter elements generally include polycrystalline diamond compact (PDC), thermally stable polycrystalline (TSP), and natural diamond.
A drag-type drill bit typically includes a shank portion with a threaded connection for mating with a drilling motor or a drill string. This shank portion can include a pair of wrench flats, commonly referred to as “breaker slots”, used to apply the appropriate torque to properly make up the threaded shank. In a typical application, the distal end of the drill bit is radially enlarged to form a drilling head. The face of the drilling head is generally round, but may also define a convex spherical surface, a planar surface, a spherical concave segment, or a conical surface. In any of these applications, the body includes a central bore open to the interior of the drill string. This central bore communicates with several fluid openings in the bit used to circulate fluids to the bit face. In typical drill bit construction, nozzles situated in each fluid opening control the direction and flow of drilling fluid.
Typically, the drilling head or bit body of a drag-type drill bit is made from a steel or a cast matrix provided with cutting elements having a layer of super-hard material. Prior art steel-bodied bits are machined from steel and typically have cutting elements that are press fit or brazed into pockets provided in the face of the bit body. Cutters are typically mounted in steel-bodied bits by brazing directly into the pockets provided in the bit face.
Cast matrix drill bits are conventionally manufactured by casting the matrix material in a mold configured to give a bit body the desired shape. Such matrixes can, for example, be formed of a copper-nickel alloy containing powdered tungsten carbide. Matrixes of this type are commercially available to the drilling industry. The cutting elements for the matrix bit body are typically formed from polycrystalline diamond compact (PDC) or thermally stable polycrystalline diamond (TSP) PDC cutter elements are brazed in an opening provided in the matrix body, while TSP cutters are cast within pockets provided in the matrix body.
The cutting action in prior art bits is primarily performed by the outer semi-circular portion of the cutting elements. As the drill bit is rotated and downwardly advanced by the drill string, the cutting edges of the cutter elements will cut a helical groove of generally semi-circular cross-sectional configuration into the face of the formation. When drilling bore holes into subterranean formations, conditions are often encountered where the drill bit passes readily through a comparatively soft formation and then strikes a significantly harder formation. Rarely do all the cutters on a conventional drag-type drill bit strike this harder formation at the same time. A substantial impact force is therefore incurred by the one or two cutters that initially strike the harder formation. The end result is high-impact load on cutter elements of the drill bit. Moreover, substantial wear or uneven destruction of the cutters initially striking the harder formation lessens the drill bit life.
The general theory of drag bit operation is to create tiny fractures as the cutting elements pass over the formation, thereby enabling drilling fluid to enter these fractures and remove the fractured portions of the formation. While most drag-type drill bits use this crushing or fracturing action to create a bore hole, some bits have been developed utilizing a shearing action to cut through the formation. Drill bits are generally designed to cut the earth formation to a desired three-dimensional profile which generally parallels the configuration of the operating end of the drill bit.
“Side rake”, a term applied to the position of the cutting faces of a cutting element with respect to the bit body, is technically defined as the complement of the angle between (1) a giving cutter face and (2) a vector in the direction of motion of the cutting face while in use, the angle being measured in a plane tangential to the earth formation profile at the closest adjacent point. “Back rake”, another term used to define the relative position of the cutting face of a cutting element with reference to the supporting bit body, is defined as the angle between (1) the cutting face of the cutting element; and (2) the normal to the earth formation profile at the closest adjacent point, measured in a plane containing the direction of motion of the cutting member, for example, a plane perpendicular to both the cutting face and the adjacent portion of the earth formation profile.
Proper selection of the back rake angle is particularly important for efficient drilling in a given type of earth formation. In soft formations, relatively small cutting forces may be used so that cutter element damage problems are minimized. However, in hard formations, significant back rake angles are utilized in order to avoid excessive wear in the form of breakage or chipping of the cutting elements due to the higher cutting forces.